The present invention relates to rotary drilling and, more particularly, to a directional drilling technique for providing deviated wellbores at significantly greater inclinations and/or over horizontal distances substantially greater than that currently being achieved by conventional directional drilling practices. The success of such directional drilling should benefit mainly offshore drilling projects as platform costs are a major factor in most offshore production operations. Wellbores with large inclination or horizontal distance offer significant potential for (1) developing offshore reservoirs not otherwise considered to be economical, (2) tapping sections of reservoirs presently considered beyond economical or technological reach, (3) accelerating production by longer intervals in the producing formation due to the high angle holes, (4) requiring fewer platforms to develop large reservoirs, (5) providing an alternative for some subsea completions, and (6) drilling under shipping fairways or to other areas presently unreachable.
A number of problems are presented by high angle directional drilling. In greater particularity, wellbore inclinations of 60.degree. or greater, combined with long sections of wellbore or complex wellbore profiles, present significant problems which need to be overcome. The force of gravity, coefficients of friction, and mud particles settling are the major physical phenomena of concern.
In the rotary drilling of a highly deviated wellbore into the earth, a drill string comprised of drill collars and drill pipe is used to advance a drill bit attached to the drill string into the earth to form the wellbore. As the inclination of the wellbore increases, the desired weight-on-bit for effective drilling from the drill string decreases as the cosine of the inclination angle, and the weight of the drill string lying against the low side of the wellbore increases as the sine of the inclination angle. The force resisting the movement of the drill string along the inclined wellbore is the product of the apparent coefficient of friction and the sum of the forces pressing the string against the wellbore wall. At an apparent coefficient of friction of approximately 0.58 for a common water base mud, drill strings tend to slide into the wellbore from the force of gravity at inclination angles up to approximately 60.degree.. At higher inclination angles, the drill strings will not lower from the force of gravity along, and must be mechanically pushed or pulled, or alternatively, the coefficients of friction can be reduced.